The influence of die tilting on the thermal response and die attach stress of a bottom exposed package

In this paper, a study on thermal and mechanical performance of a power discrete package with different die sizes and non-uniform bond line thickness (BLT) variations is conducted by finite element analysis and experiment. Two different die sizes of 4.14×3.04×0.2mm (big die) and 1.78×0.88×0.2mm (small die) in IPAK package are used in this study to exam steady state junction-to-case thermal resistance (R_thJC). Meanwhile, the effect of die tilting variations on R_thJC is also analyzed. Based on the results, the influence of the die tilting on R_thJC is 10% and 8% between non-tilting and worst tilting on the big die and the small die respectively. The difference of R_thJC from experiment shows the quit close results. Transient thermal performance on different die tilting variations is also studied. The results show that the worst tilting case has about 8% larger R_thJC in junction temperature than non-tilting case. In addition to the thermal resistance, the Von Mises stress of soft solder used for die attach for the worst tilting case is 15% more than the non-tilting case, and the largest stress is located in the area where the solder becomes thinnest. The stress exceeds the solder tensile strength of 26-34Mpa in both cases. The worst tilting case has 2.5 times more plastic strain than the non-tilting case, and the largest strain is also concentrated in the thin solder area. The strain value is still bellow the elongation limitation of 0.23-0.33. While the TC500 testing for this two groups shows both chip and solder are safe.